A stent is a medical device used for improving a stenosed region or an occluded region in a lumen in an organism such as a blood vessel, a bile duct, a trachea, an esophagus, an urethra or the like. Generally speaking, the stent is a network-like cylindrical body formed by successively forming, using a thin strut, a straight or curved linear portion or a bent portion of a U shape or the like on the same plane, so to speak, into a wavy form and disposing a plurality of wavy annular bodies, in each of which such wavy struts are disposed annularly and connected to each other, in an aligned relationship with each other in an axial direction.
While a stent is used, for example, in a coronary artery of the heart, for restenosis prevention after the percutaneous transluminal coronary angioplasty (PTCA), it is recognized that, if a stent on which drug is not coated (i.e., a bare metal stent) is used such that the stent is reduced in size in advance and reaches a target region, in which it is expanded so as to be indwelled in a lumen, although the restenosis rate is low in comparison with a case where only PTCA is used without using a stent at all, restenosis occurs at a ratio of approximately 20 to 30% in the stent indwelling region. Principal causes of the restenosis are intimal hypertrophy by migration and growth of vascular smooth muscle cells.
Accordingly, development of a DES is carried out recently in which a drug capable of suppressing migration and growth of vascular smooth muscle cells is coated on the outer surface of a stent (referred to sometime as “stent body”) such that the drug is eluted in a stent indwelling region to prevent restenosis. For example, the drug used include sirolimus and carcinostatic. The coating of the drug is carried out such that a coating liquid including a drug and a biocompatible polymer which are dissolved in solvent is coated by a dipping method, a spray method, or a direct application method (applying drug along a strut which configures a stent body) or the like so that a predetermined amount of drug exists on the surface of the stent body and then the coating liquid is dried and solidified.
However, in order to cause the DES to be indwelled in a lumen, after the stent reaches a target region in a lumen in a state in which the stent is reduced in diameter once, the stent is expanded and put so as to be indwelled. It has been found that a problem occurs in that the drug coat layer coated on a bent portion or the like is exfoliated from the surface of the stent body following the expansion and deformation of the stent (particularly in the bent portion) and the drug itself is destroyed and comes off from the stent body. This problem is particularly acute where the drug coat layer exhibits brittleness.
International Application Publication No. WO 03/009779A2 discloses forming a drug coat layer in the form of a line or in the form of a dot in a region other than a bent portion (“stress region” or “mechanical profile”) of the stent or form a drug coat layer in the form of a dot at a bent portion.
However, there is the possibility that, in a stent in which a drug coat layer is formed in the form of a dot, the amount of the drug may be insufficient, and this is not preferable. Further, even in a stent in which a drug coat layer is formed in the form of a line at a portion other than the curved portion, there is the possibility that a start point of deformation by expansion may coincide with an end portion of the drug coat layer or, depending upon a case, a start point of deformation may exist in a portion in which the drug coat layer is formed. Therefore, there is the possibility that strain or stress concentration may occur in the drug coat layer and the drug may be exfoliated from the surface of the stent body and then the exfoliation may propagate further to destroy the drug coat layer and cause removal of a significant amount of the drug coat layer. In particular, where the stent is expanded in a radial direction, the magnitude or the like of expansion is not constant and is different for each procedure, and coating of the drug is carried out for a bent portion to be expanded without taking which position becomes a start point of deformation into consideration. Therefore, even if coating is carried out uniformly except at a bent portion, if a start point of deformation and an end portion of the drug coat layer coincide with each other, then strain or stress concentration occurs in the drug coat layer and exfoliation, destruction and coming off of the drug occurs.